(1) Field of the Invention
The invention relates to compressors. More particularly, the invention relates to compressor unloading in air conditioning or refrigeration systems.
(2) Description of the Related Art
In a closed air conditioning or refrigeration system there are a number of methods of unloading that can be employed. Commonly assigned U.S. Pat. No. 4,938,666 discloses unloading one cylinder of a bank by gas bypass and unloading an entire bank by suction cutoff. Commonly assigned U.S. Pat. No. 4,938,029 discloses the unloading of an entire stage of a compressor and the use of an economizer. Commonly assigned U.S. Pat. No. 4,878,818 discloses the use of a valved common port to provide communication with suction for unloading or with discharge for Vi control, where Vi is the discharge pressure to suction pressure ratio. In employing these various methods, the valve structure is normally fully open, fully closed, or the degree of valve opening is modulated so as to remain at a certain fixed position. Commonly assigned U.S. Pat. No. 6,047,556 (the '556 patent, the disclosure of which is incorporated by reference herein as if set forth at length) discloses the use of solenoid valve(s) rapidly cycling between fully open and fully closed positions to provide capacity control. The cycling solenoid valve(s) can be located in the compressor suction line, the compressor economizer line and/or the compressor bypass line which connects the economizer line to the suction line. The percentage of time that a valve is open determines the degree of modulation being achieved.
Nevertheless there remains room for further improvement in the art.